1. Field of the Invention
This invention relates in general to audible alerting mechanisms. In particular, the invention discloses a technique for controlling the volume of sound generated by a magnetic transducer, such as those used for producing ringing tones in a telephone system signaling an incoming telephone call.
2. Background Art
Telephone receiver products typically include audible ringers to alert users to the presence of an incoming call. Such telephone ringers often employ a form of magnetic transducer to convert an electrical ringing signal into an audible tone. Because telephones are ubiquitous, and used in a wide range of physical environments, most telephones also provide for the user to be able to control the ringer volume produced by the magnetic transducer. As a result, telephones can be used effectively in noisy environments, such as a factory or warehouse, where a high ringer volume is required to ensure that incoming call signals can be heard, as well as in quiet environments, such as an individual office, where a low volume is sufficient to adequately alert the office occupants to an incoming call. Adjustment of ringer volume also allows for the selection of a wide range of personal preferences as to the desired ringer volume.
Many conventional ringers produce their sound by driving a transducer with a square wave signal. One technique for controlling the volume of such a ringer is to vary the amplitude of the square wave signal. This technique is depicted in FIG. 1. FIG. 1(a) depicts a full volume square wave, while FIG. 1(b) depicts a reduced amplitude square wave. However, amplitude control requires that the telephone set include a circuit that produces a driving signal with a variable amplitude. Such circuits typically require an analog driver stage subsequent to the driving signal generator, thereby introducing additional circuit components to the telephone design that would not be necessary if the transducer were driven, for example, solely by a digital controller generating a square wave generated by mere toggling of a digital logic line. These additional analog components increase both the size and the cost of the circuit. Even if a telephone set is designed using components such as an application specific integrated circuit (ASIC), integration of an analog driver section may result in an ASIC with larger die size, more complex design, and reduced reliability than would be the case for a purely digital design. Therefore, it is an object of the invention to provide a ringer with volume control that does not require an analog variable-gain driver.
Another method for controlling the volume of a ringer signal is pulse-width-modulation (PWM), which use results in the signal depicted in FIG. 1(c). This technique produces reduced volume by reducing the pulse width of the driving signal. While PWM provides an entirely digital solution to ringer volume control, the disadvantage of this method is that the timbre of the ringer signal (i.e. its harmonic content) changes as the width of driving signal pulses is varied. To the user, this characteristic causes lower volumes to sound “tinnier” than higher volumes, since low frequency components of the signal are attenuated more than high frequency components when pulse width is reduced. Therefore, it is also an object of this invention to provide a circuit with improved consistency in the tone quality of a ringer sound over a range of ringer volumes that can be implemented as a digital circuit.
These and other objects of the present invention will become apparent in light of the present specifications and drawings.